Boron coatings on graphite substrates have many useful applications. With its thermal and chemical stability, boron can be employed in the ion sources of ion implantation machines used in the manufacture of semiconductor devices. Particularly in the case of boron ion implantation, a boron or boron-coated ion source is capable of providing higher beam current, higher beam purity and lower erosion rates than ion sources made of refractory metals such as tungsten or molybdenum. Other components in ion implantation machines, such as beam dumps, could also advantageously be made of boron or boron-coated materials. Other applications occur in nuclear devices where the high temperature stability and large neutron cross-section of boron can be used in shielding and in reactor walls.
Pure elemental boron is difficult to fabricate into such components by ordinary means. The pure element is refractory and brittle, and near its melting point, boron has a very high vapor pressure. Hence, the usual metal fabrication process of casting and machining cannot be used to fabricate components. Neither can ceramic processing techniques be applied readily. Hot-pressing of boron powders to fabricate plates or shapes leads to high residual stresses which result in immediate or eventual failure during use.
Chemical vapor deposition (CVD) is a practical method for forming boron coatings. The manufacturing of CVD boron fibers is a well-known technology. Boron-coated fibers prepared by CVD processes are in widespread use. For example, a description of one process of application of a boron-based refractory metal on a silicon carbide filament is taught by U.S. Pat. No. 4,481,257, which patent is incorporated herein by reference. In a typical manufacturing process, a small diameter substrate wire, typically tungsten or carbon, is heated in the presence of a boron halide and hydrogen. The boron halide is reduced and elemental boron deposits on the substrate. Tungsten substrate wires are typically in the range of 10 to 12 microns in diameter, while carbon substrate wires are typically somewhat thicker in the range of 25 to 50 microns in diameter. The resulting boron-coated fibers are in the range of 100 to 200 microns in diameter.
The components required for semiconductor applications, for example, have dimensions much larger than those of the fibers, for example in the range of 5 to 15 centimeters. Application of fiber CVD technology to the formation of boron coatings on these substrates does not provide suitable results. The boron coatings exhibit multiple cracks and tend to spall off the substrates. A major reason for this is a mismatch of coefficients of thermal expansion (CTE) between the substrate and the boron coating. The coatings are deposited at temperatures in excess of 1000xc2x0 C. Upon cooling to ambient temperature, the differential shrinkage between the substrate and coating results in stresses which crack the coating and/or result in lack of adhesion (i.e. produce a fracture at the interface between the coating and the substrate). This is not a problem in the CVD formation of small-diameter fibers because the thermally-induced strains and the resultant stresses are too small to cause this type of fracturing. For larger substrates, thermal stresses can be minimized by selecting a substrate material which has the same thermal expansion characteristics as those of boron.
While the use of graphite substrates which match the CTE of boron eliminates the macroscopic cracking observed with other substrates, the boron coatings produced on these graphite substrates by CVD technology are still deficient as they exhibit a high concentration of voids near the interface with the graphite substrate. In some circumstances, the voids may coalesce, producing large voided regions. Such voided regions are prone to fracture resulting in microscopic cracks and in the coating spalling off the substrate.
It is well known that the surface of a graphite article can be sealed by the application of a smooth sealing coating of pyrolytic graphite. While such smooth sealing coatings seal the surface porosity of the underlying graphite, they do not provide an appropriate surface for the CVD deposition of boron on large surfaces because the boron coatings generally are not adherent to these smooth coatings and tend to spall off the sealing coatings. It is hypothesized that the mechanical locking which occurs between the surface roughness of unsealed graphite and the conformal boron deposit is an important factor in the adhesion of the boron coatings.
For all of these reasons, prior art techniques for coating boron onto graphite substrates and the like have been unsatisfactory or had only limited utility. The improved substrate treatment and coating method of the present invention overcomes many or all of the problems with and deficiencies of the prior art techniques.
In the improved substrate treatment and coating method of the present invention, the surface of a graphite or comparable substrate that is to be coated with boron is first densified with carbon to reduce surface porosity while still retaining sufficient surface texture to enhance the adherence of the subsequently applied boron coating.
In one embodiment of the present invention, referred to herein as a rapid densification process, a relatively porous graphite substrate is immersed in a liquid hydrocarbon. While under immersion, the substrate is heated by suitable means to a temperature above the decomposition temperature of the hydrocarbon. This creates a temperature gradient through the substrate, the hottest portion being in the body interior, and the coolest being at the surface of the substrate. As hydrocarbon vapors diffuse into the substrate interior through the pores, they reach the hottest portion in the interior of the body, where the temperature exceeds the decomposition temperature of the hydrocarbon. At this temperature, the vapors decompose and deposit solid carbon in the pores. The resultant substrate is thereby densified and prepared for subsequent boron deposition.
In a second invention embodiment, chemical vapor infiltration of the substrate pores with a gaseous hydrocarbon is employed to prepare the substrate surface by at least partially filling the surface porosity, while preserving adequate surface texture for good adherence of a subsequently applied boron coating. Under appropriate conditions identified below, excellent substrate bonding surfaces are achievable with both embodiments of this invention.